This is WEEKEND EDITION from NPR News. I'm Scott Simon. Earlier this week, the World Health Organization said the Ebola epidemic was still growing and that it could afflict thousands of more people before it's brought under control. But there is one sign of some progress. The National Institutes of Health will begin human trials of an experimental Ebola vaccine starting next week. Dr. Anthony Fauci is director of the National Institute of Allergy and Infectious Diseases at the NIH. Dr. Fauci, thanks so much for being with us.

DR. ANTHONY FAUCI: Good to be with you.

SIMON: What goes into a trial like this? How big is it?

FAUCI: Well, it starts off small by its very nature; that it's what we call a Phase 1 trial. This is a vaccine candidate that has been tested in a nonhuman primate monkey model and shown to be quite effective in preventing infection in monkeys who were given a lethal challenge of Ebola. And once you establish it in a monkey model, the next obvious phase is to see first, is it safe to give to humans - because this is the first time that this vaccine has been in humans. And number two, does it induce the kind of response that you would predict would be protective?

So you really want to be so careful because safety is paramount when you do these kinds of studies. So we start off with very few people. The total number of people will be normal volunteers, aged 18 to 50, to a number of 20. And you start off slowly with three. You give it to them. You observe them for a while at a low dose. And then you work your way up to a higher dose until you get to the total of 20. And then you evaluate - was it safe? Were there any kinds of reactions that would be worrisome? And does it induce the kind of response you want it to?

SIMON: And let's try and understand this clearly. This is a preventive vaccine; not a cure.

FAUCI: Absolutely. And that's an important thing to emphasize. What we're talking about now is a classical vaccine, very similar to when you vaccinate a child against measles or mumps or rubella. It's to prevent the infection. And that's what we're talking about with this vaccine, is to prevent Ebola infection. And it will be much more targeted towards the health care workers who are in the West African countries who are at considerable risk. So although we hope if the vaccine is successful that it could be used on a broader way, the first people you want to get are the first responders; the people who are there on the front line taking care of people.

SIMON: Why have this trial in suburban Maryland, which is not a hotbed of Ebola, rather than Liberia or Sierra Leone?

FAUCI: That's very standard that when you start off the first time in humans, you want to get healthy individuals under the very well-controlled circumstances of being able to very carefully monitor laboratory tests, any symptoms, anything that might indicate a hypersensitivity or an adverse reaction. It's very difficult to do that within the countries that are now experiencing this outbreak because of the lack of infrastructure to conduct these types of very carefully monitored trials. We're negotiating right now with the Health Ministry of Nigeria, which does have a much better infrastructure than the three other countries of Sierra Leone, Guinea and Liberia, to see if we can also simultaneously do a Phase 1 trial under the better conditions that are there. But we have not worked that out yet.

SIMON: And Dr. Fauci, if the results seem to be positive, how long before these vaccines might actually be approved and implemented in West Africa?

FAUCI: The standard way you go about getting approval of a vaccine like this would be to finish the Phase 1 trial, which in this case, we're starting the day after Labor Day and it will be likely until the end of the calendar year that we'll have those results. If it looks safe and it induces the kind of response that we hope it is, then you would generally proceed to a larger trial to determine in fact if it does work. That should likely take place some time in the first quarter or the first half of 2015.

The other alternative is, if you could do some sort of an expanded access to people at the same time that you're conducting a clinical trial. So it really - how is the situation going to be in the beginning of the year? Will the epidemic or outbreak be under control or not? All of that dictates how you're going to be able to approach the next stage of this vaccine trial.

SIMON: Dr. Anthony Fauci is director of the National Institute of Allergy and Infectious Diseases at the NIH. Thanks very much, Doctor.

FAUCI: Good to be with you. Transcript provided by NPR, Copyright NPR.